It is often necessary for a person who is not a medical professional, to evaluate the condition of a weak patient who has signs of cardiopulmonary arrest, especially when the patient is partly or completely unconscious. One of the first steps of a rescuer is to determine if there is sufficient breathing and adequate blood circulation. Unfortunately, evaluation of these two parameters is difficult. For identification of blood circulation by detecting a pulse, a common method is to place a hand on a region of the body (e.g. carotid artery) and feel for small fluctuations. For evaluation of breathing, a common method is to place the rescuer""s face near the mouth to feel or hear the flow of air or movement of the chest. Although detection of pulse and breathing is not difficult in normal patients, such identification is difficult in weak patients, such as a patient in shock, where there is low flow of blood and of air. The inability to detect pulse and respiration at this time increases the probability of an incorrect diagnosis between cardiac arrest, breathing arrhythmia (irregular heartbeat), or asphyxia. The consensus of investigators of cardiopulmonary resuscitation is that current xe2x80x9cpulse checkxe2x80x9d has less than a fifty percent accuracy in identifying cardiac arrest.
An apparatus and method that facilitated a determination of the cardiac and pulmonary functions of a patient, by persons who are not medical professions, and by noninvasive means, would be of value.
Two other parameters useful in medical diagnoses by medical professionals, is the blood pressure (both systolic and diastolic) and the volume of air in each breath. For normal patents (those who are not weak) blood pressure can be taken by a cuff that fits around the arm, and breathing volume can be measured by having a patient breath out into a container. This is not possible for a weak patient. An apparatus and method that enabled such measurements by noninvasive means, would be of value.
In accordance with one embodiment of the present invention, a method and apparatus are provided for evaluating the condition of a patient who is weak, such as one who has symptoms of cardiopulmonary arrest. Alternating current of a frequency between about 1 kHz and 90 kHz is applied between two electrodes lying at opposite sides of the chest of the patient. Variations in voltage are measured to detect variations in impedance of the patient""s chest area. Such variations are primarily due to activity of the heart and respiration system. An analyzing circuit determines the average amplitude and frequency of those signals representing heartbeats, and preferably also determines the amplitude and frequency of signals representing respiration.
If the respiratory rate is less than about 4 breaths per minute and the heart rate is less than about 20 beats per minute, then this indicates cardiac arrest. A care giver who is ready to perform CPR (cardiopulmonary resuscitation) can use the information about the patient""s status, to emphasize chest compressions to simulate cardiac activity, or concentrate on applying quantities of air to the patient""s lungs to emphasize breathing. The fact that heartbeats and/or breathing can be detected, provides encouragement to the care giver.
A separation of the signal components representing cardiac activity from those representing respiration, and from those representing extraneous signals, is accomplished by filtering the demodulated alternating current passing through the patient""s chest. Those frequencies above about 0.3 Hz (18 per minute) represent cardiac activity. The portion of the demodulated signal representing respiration can be separated from that representing cardiac activity, by filtering out frequencies below about 0.25 Hz (15 breaths per minute) since a weak patient such as one having signs of cardiopulmonary arrest breathes at less than 15 breaths per minute.
The apparatus can indicate no cardiac arrest when the respiratory rate is above about 4 bpm (breaths per minute) and the heart rate is above about 20 bpm (beats per minute). A respiratory rate above about 4 bpm and heart rate less than about 20 bpm indicates no cardiac arrest but extreme breathing arrhythmia. A respiratory rate less than about 4 bpm and heart rate above about 20 bpm indicates no cardiac arrest, but possible asphyxia. A respiratory rate less than about 4 bpm and heart rate less than about 20 bpm indicates cardiac arrest.
The detected signals representing cardiac activity can be used to determine the blood pressure (systolic and diastolic) of the patent. The detected signals representing respiration can be used to determine the volume of each breath of the patient.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.